Knowledge of the molecular mechanisms involved in chemo-mechanical transduction and the regulatory elements determining contraction and relaxation is largely derived from cross-striated muscle. However, vertebrate smooth muscles have features lacking in striated muscle. These characteristics reflect regulated crossbridge cycling rates observed mechanically as variable shortening velocities. We have proposed that covalent regulation involving Ca++-dependent phosphorylation of crossbridges in smooth muscle can predict the mechanical properties with the same basic molecular motor depicted in the classic sliding filament/crossbridge paradigm developed for striated muscle. Muscles can also be characterized in energetic terms. Relatively little information is available about the energetics of smooth muscle, and some of the published data conflicts with the predictions of our 4-state crossbridge hypothesis developed to explain the mechanical output. One aim of the project is to experimentally determine key energetic parameters, such as ATP consumption, economy of force generation, and the efficiency of work performance as a function of crossbridge phosphorylation. This will provide basic information about the properties of phasic and tonic smooth muscles and allow assessment of theories for regulation and chemo-mechanical transduction. An implicit aspect of covalent regulation is that conventional assessments of smooth muscle activation can be misleading. Given the effort directed to assessing drug, hormone, and neurotransmitter actions on smooth muscles, another aim is to define the determinants of activation and identify appropriate methods for the quantitative assessment of smooth muscle activation. The last objective is part of a long range effort in the program to develop vascular organ culture models where the phenotype can be modulated and the functional effects measured.